Within the IP (Internet Protocol) Multimedia Subsystem (IMS) as defined by 3rd Generation Partnership Project (3GPP) Session Initiation Protocol (SIP) defined by the Internet Engineering Task Force (IETF) is used for controlling communication. SIP is an application-layer control protocol for creating, modifying, and terminating sessions with one or more participants. These sessions may include Internet multimedia conferences, Internet telephone calls, and multimedia distribution. Members in a session can communicate via multicast or via a mesh of unicast relations, or a combination of these. Session Description Protocol (SDP) is a protocol which conveys information about media streams in multimedia sessions to allow the recipients of a session description to participate in the session. The SDP offers and answers can be carried in SIP messages. Diameter protocol has been defined by IETF and is intended to provide an Authentication, Authorization and Accounting (AAA) framework for applications such as network access or IP mobility.
Generally, for properly establishing and handling a communication connection between network elements such as a user equipment and another communication equipment or user equipment, a database, a server, etc., one or more intermediate network elements such as control network elements, support nodes, service nodes and interworking elements are involved which may belong to different communication networks.
The IMS is the standardized solution for multimedia telephony over IP based networks, e.g. voice for Long Term Evolution (VoLTE) is utilizing the IMS, which thus receives increased interest from the operator community. There exists related standardization activity focusing on the SIP based interconnection of IMS networks. In particular 3GPP specification TS 29.165 describes the SIP profile applicable at the Inter-IMS Network to Network Interface (II-NNI). Related discussions within the 3GPP operators association, GSMA, 3GPP studied a “Roaming Architecture for Voice over IMS with Local Breakout” (RAVEL) in specification TR 23.850, and “Optimised Service Charging and Allocation of Resources in IMS whilst Roaming” (OSCAR) in specification TR 23.849.
As result of recent discussions, various II-NNI interfaces can be encountered for a call between a calling IMS User Equipment (UE) UE-A, which is roaming in a visited Public Land Mobile Network (V-PLMN) A and has a subscription in a home PLMN (H-PLMN) A, and a called UE B roaming in a V-PLMN B and with a subscription in a H-PLMN-B, as shown in FIGS. 1-3.